Yogurt

Yogurt is basically a form of curdled milk much like sour cream and creme fraiche, but with less fat.

Bacteria in the milk ferments and coagulates to thicken the milk to a creamy texture, adding a tangy, slightly astringent flavor. In commercial manufacturing, the friendly bacteria is added, but if you were to heat fresh milk and keep it at about 100°F. for a few hours, it would naturally turn to yogurt.